1. Field of the Invention (Technical Field)
The present invention relates to methods and apparatuses for controlling vacuum arc remelting (VAR) furnaces.
2. Background Art
Vacuum arc remelting (VAR) is a process used to control the solidification of segregation sensitive alloys. A simplified schematic of the process is shown in FIG. 1. A cylindrically shaped, alloy electrode 1 is loaded into the water-cooled, copper crucible 2 of a VAR furnace, the furnace is evacuated, and a direct current (dc) electrical arc is struck between the electrode (cathode) and some start material (e.g., metal chips) at the bottom of the crucible (anode). The arc heats both the start material and the electrode tip, eventually melting both. As the electrode tip is melted away, molten metal drips off, forming an ingot 3 beneath while the electrode is consumed. Because the crucible diameter is typically 50-150 mm larger than the electrode diameter, the electrode must be translated downwards toward the anode pool to keep the mean distance between the electrode tip and pool surface constant; this mean distance is called the electrode gap (g.sub.e) 4. As the cooling water 5 extracts heat from the crucible wall, the molten metal next to the wall solidifies. At some distance below the molten pool surface, the alloy becomes completely solidified, yielding a fully dense ingot. After a sufficient period of time has elapsed, a steady-state situation evolves consisting of a "bowl" of molten material situated on top of a fully solidified ingot base. As more material solidifies, the ingot grows.
The other significant parts of a typical VAR furnace shown in FIG. 1 include vacuum port 6, furnace body 7, cooling water guide 8, ram drive screw 9, and ram drive motor assembly 10.
The success of the VAR process depends on its ability to: (a) continually supply the advancing solidification region with liquid metal while minimizing the local solidification time, t.sub.s ; and (b) provide a stable steady-state thermal environment for the solidification process. If t.sub.s becomes too long (deep pool, relatively low longitudinal thermal gradient), alloying elements will have time to segregate near the ingot center and the probability of macrosegregation will increase. Therefore, it is necessary to carefully control pool depth in segregation sensitive alloys. However, if a stable melting environment is not achieved, neither the pool depth nor its composition will be stable, and the probability of process failure through the formation of solidification defects will be greatly enhanced.
One parameter that is crucial to the stability of the VAR process is electrode gap. If g.sub.e becomes too large, the arc will search for a less resistive path to ground, the result being intermittent arcing directly to the crucible wall. This often causes a reduction in process efficiency, but more importantly it gives rise to an unstable energy input into the ingot pool surface. If the condition persists, the electrode tip will become rounded, all of the molten metal from the electrode will drip into the center of the pool, and the pool will begin to freeze in from the sides. This constitutes a severe disruption of the process. If g.sub.e becomes too small, transient arc interruptions occur due to multiple, nearly simultaneous contacts between the electrode and ingot surfaces. This, too, leads to decreased melt rate, process instability, and severe disruption of the solidification process. However, it is not sufficient merely to control g.sub.e within the range where it is neither too small nor too large, but it must be controlled at a constant value within this range if process stability is to be achieved.
Most modern low current (&lt;10 kA) VAR controllers use drip-short frequency (f.sub.DS) or period (1/f.sub.DS) to control electrode gap. Though three patents were issued in the last decade associated with various forms of drip-short control (U.S. Pat. Nos. 4,303,797, to Roberts; 4,578,795, to Fisher et al.; and 4,797,897, to Stenzel et al.), the basic phenomenon was discovered in the late 1950's and a drip-short based VAR control system was patented in 1960 by Johnson (U.S. Pat. No. 2,942,045). The basic drip-short phenomenon has been carefully investigated, see, e.g., F. J. Zanner, "Metal Transfer During Vacuum Consumable Arc Remelting," Metallurgical Transactions B, Vol. 10B, 1979, pp.133-42, and R. L. Williamson et al., "Voltage Signatures In VAR," Proceedings Vacuum Metallurgy Conference, N. Bhat, et al., eds., Iron and Steel Society, Warrendale, Pa., 1992, pp. 87-91, and may be described as follows. As molten metal drips from the electrode surface, it sometimes comes in contact with the anode pool before separating, forming a molten metal bridge between the two surfaces. This causes the melting current to momentarily flow through the bridge, giving rise to a sudden decrease in the arc voltage. The disruption usually lasts for 10.sup.-4 -10.sup.-3 seconds before the bridge ruptures and arcing resumes. There is a characteristic voltage signature associated with these "drip-shorts." The number of these events that occur per second, f.sub.DS, is a function of g.sub.e. If a sufficient number of events (circa 100) are counted to give a statistically meaningful value, f.sub.DS may be used as a control parameter to accurately predict and adjust electrode gap. In commercially available controllers, a drip-short frequency or period set-point is entered in and the ram drive motor 10 speed is adjusted to achieve the set-point directly. No estimate of g.sub.e is made and it is assumed that constant f.sub.DS implies constant electrode g.sub.e.
As melting current is increased to obtain higher melt rates, f.sub.DS decreases concomitantly. F. J. Zanner, et al., "Relationship between Furnace Voltage Signatures and the Operational Parameters Arc Power, Arc Current, CO Pressure, and Electrode Gap during Vacuum Arc Melting of INCONEL 718," Metallurgical Transactions B, Vol. 17B, 1986, pp. 357-65. One reason for this involves measurement electronics: as the melting current is increased, the molten metal contact is interrupted more quickly due to increased Joule heating and the measurement electronics have to be faster to capture all of the events. More important, however, is the fact that fewer drips form at high melting currents because increased agitation of the electrode tip surface interrupts the formation mechanism. At high melting currents (&gt;20 kA), drip-shorts are only observed at very tight electrode gaps. Hence, at medium-to-high melting current (10-40 kA), drip-short based control becomes less effective (or wholly ineffective) and arc voltage is usually used as an indicator of g.sub.e. As with drip-short based VAR control systems, modern voltage-based control systems do not directly control g.sub.e. The ram drive motor speed is varied to achieve a voltage set-point, not a g.sub.e set-point; constant arc voltage is assumed to ensure constant g.sub.e. At low melting current, the sensitivity of arc voltage to changes in g.sub.e is not sufficient to make voltage control a viable technique. Direct voltage control has been practiced since the 1950's.
Another means of controlling electrode gap during the VAR process is to adjust electrode ram speed in response to melt rate (see U.S. Pat. No. 4,131,754, to Roberts). Obviously, as melt rate increases (decreases), the electrode gap must open (close) if ram speed is not changed. Melt rate control is usually not used by itself to control electrode gap because of: (1) lack of precision in electrode weight measurement using load cells during melting; and (2) inherent inaccuracies in the measured electrode and crucible parameters used to relate melt rate to electrode gap. This problem may be partially alleviated by long term (about 20 minutes) averaging of load cell data; however, this causes the system to be highly damped and unresponsive to process transients. To address this problem, Roberts developed a means of VAR electrode gap control wherein melt rate is used to establish the base electrode feed rate and drip-short period is used to trim the feed rate (U.S. Pat. No. 4,303,797). The patent suggests that a simple two-input electrode gap control system eliminates response problems by combining a relatively fast, accurate control signal (drip-short period) with the melt rate signal. This comprises a melt rate controller that is continually being corrected with drip-short based information. This is no improvement over straightforward drip-short based control systems. Roberts states that these, by themselves, tend to over-respond to process transients. However, this is not considered a deficiency in modern drip-short based electrode gap control systems. As will be demonstrated below, there are significant advantages to be gained by using multiple input control systems, but only if the inputs are combined in such a fashion as to yield direct and optimal estimates of electrode gap.
Other examples of methods to control VAR furnaces include: U.S. Pat. Nos. 4,775,981, to Kohnert et at.; 3,872,231, to Motter et al.; 3,385,920, to Harbaugh et al.; 3,381,079, to Murtland et al.; 3,375,318, to Kj.o slashed.lseth et al.; 3,364,295, to Roberts; 3,277,229, to Oppenheim; 3,187,078, to Murtland et al.; 3,186,043, to Murtland et al.; 3,143,587, to Buehl; 2,915,572, to Buehl; and 2,904,718, to Cooper et al.
The VAR process is complex and there is no known gap control method that is completely foolproof. Regardless of the method used, conditions may, and often do, arise where significant gap excursions occur while holding the controlled variable (arc voltage, drip-short frequency or melt rate) constant. Hence, effective electrode gap control requires the detection of process anomalies, or "upsets," as well as methods for their mitigation. Common examples of VAR process anomalies are "glow," electrode tip geometry changes, and melt rate excursions due to cracks and/or arc constrictions. During a glow condition, arc voltage and melt rate decrease, and drip-shorts cease. This causes a voltage-based gap estimate to be too small and a drip-short estimate to be too large, making for an unpredictable gap estimate. Transient gap excursions to changes in electrode tip geometry are often more subtle. If the electrode tip begins to round off, the dripping dynamics change and the gap must be opened to maintain a constant f.sub.DS. Under these conditions, the mean arc voltage increases by only a relatively small amount, making for a process deviation that is difficult to detect. Melt rate excursions often cause transients in both arc voltage and f.sub.DS, giving rise to g.sub.e excursions that may be mapped into the ingot as solidification defects. Despite the fact that these types of VAR process anomalies are common in the industry, modern VAR control systems are not designed to detect and respond to them.
Additionally, attempting to control the electrode gap by controlling the drip-short frequency or arc voltage presents problems in the area of controller tuning. In general, the arc voltage and drip-short frequency are nonlinear functions of the electrode gap. Therefore, they are related to the change in electrode gap (ram velocity) in a nonlinear fashion. In order to control the drip-short frequency or arc voltage using the ram position or velocity, a nonlinear controller is needed. However, nonlinear controllers are difficult to design and analyze, since no general theory exists. If a linear controller is used to regulate drip-short frequency and voltage, the gains of the controller will be dependent on the operating conditions.
The observations that no single means of gap control is foolproof and that different types of process anomalies affect the process variables used for gap prediction in different ways point to the advantage of making multiple independent estimates of this important process parameter and combining these estimates to form a single, statistically optimal estimate. This serves to not make the successful application of the process wholly dependent upon one variable and provides means for internal consistency checks. The current generation of VAR controllers does not provide for this important capability. In the specialty metals industry, there is a demonstrated need for a linear VAR electrode gap controller that uses multiple inputs to form optimal electrode gap estimates, that controls the parameter of interest instead of the variable used to estimate it, and that has the ability to detect and react to anomalous process behavior. The objective of this invention is to demonstrate a general, linear, VAR electrode gap control method and apparatus that uses all available information to predict and control electrode gap while detecting, logging and appropriately reacting to common process upsets.